1. Field of the Invention
The invention disclosed herein relates generally to the field of plug valves and more specifically to a bleed system for double block and bleed plug valves for avoiding dangerously high pressure build up in liquid filled, positive shut-off situations.
2. Prior Art
When a double block plug valve is closed and completely filled with liquid media, slight variations in temperature can cause drastic changes in body cavity pressure resulting from thermal expansion and media incompressibility. Valves filled with fuel oil in a laboratory setting have exhibited a 75 PSI increase in pressure with a temperature rise of only 1.degree. Fahrenheit.
While results vary under actual service conditions depending on media, pressure vessel rigidity and pressure of entrained gas, it is known that dangerously high pressure will build up in liquid filled positive shut-off valves. Therefore, double block plug valves in liquid service require a pressure relief device.
Various methods of pressure relief have been implemented in the prior art. The differential thermal relief (DTR) system is one example. A relief valve mounted at a tee on the valve bonnet, pipes excess pressure to the upstream throat of the valve. The standard relief valve is sometimes set to open at 25 PSI on all valves regardless of working pressure. With the valve closed the relief valve will open at 25 PSI above upstream pressure. This system functions only when the valve is closed. A manual body bleed valve is included. This bleed valve, installed at the tee, is opened after the valve is closed. Seal effectiveness can be immediately evaluated. This bleed valve must be closed before the valve is reopened. An isolation valve installed in the upstream throat tap is also included. It must be left open to permit the relief system to relieve pressure upstream and is closed only for repair.
A Manual Body Bleed Valve (MBBV) is another prior art example. This bleed valve installed in the body cavity is opened after the valve is closed. Seal effectiveness can be immediately evaluated. This bleed valve must be closed before the valve is reopened.
The Automatic Body Bleed Valve (ABBV) provides visual, positive assurance that the valve has sealed completely at each cycling operation and prevents thermal pressure buildup in the body cavity. An automatic bleed valve connected to the body cavity of the main valve is mechanically opened by the valve operator when the valve is closed. Seal integrity is indicated by viewing the discharge of the bleed valve. When the valve is opened, the bleed valve is automatically closed by the combination of line pressure and the spring in the bleed valve.
All of these methods protect the valve body from over-pressurization, but also increase the risk of an accident or incident due to the relative delicacy of the exposed "plumbing". As a result, more recent state of the art pressure relief systems have become quite popular. The Welded/DTR functions as a basic DTR but all pipe threads are seal welded, except for the relief valve which is installed between a set of seal welded unions. This essentially all-welded system provides enhanced incident control where safety and the environment are of the foremost concern. The Safety Bleed/DTR functions exactly as the basic DTR. However, all working components are housed in a virtually indestructible compact steel manifold. The benefits of this incident control equipment are improved fire safety, complete component access for maintenance purposes, all stainless steel components, all socket welded joints, heavy wall pipe and total incident control. However, as the pressure protection systems become more robust for enhanced incident protection they also get more costly and cumbersome.
There is therefore a continuing need for a bleed system having reduced cost and size while maintaining incident control. Moreover, such bleed systems must be fugitive emission free; must be fire safe; and must be strong and resist spillage of contents.